Modular fiber-based chirped pulse amplification system

ABSTRACT

A modular ultrafast pulse laser system is constructed of individually pre-tested components manufactured as modules. The individual modules include an oscillator, pre-amplifier and power amplifier stages, a non-linear amplifier, and a stretcher and compressor. The individual modules can typically be connected by means of simple fiber splices.

This is a continuation of application Ser. No. 10/813,163 filed Mar. 31,2004. The entire disclosure of the prior application, application Ser.No. 10/813,163 is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an ultrafast pulse source for pulseenergies in the submicrojoule to millijoule range. The present inventionuses state-of-art technologies to build an all fiber based chirped pulseamplification laser system suitable for industrial applications.

BACKGROUND OF THE INVENTION

Fiber lasers and amplifiers are promising pulse source candidates forindustrial applications, due to their unique simplicity of construction.Large core fiber amplifiers and specifically large core diffractionlimited multi-mode amplifiers (as described by M. E. Fermann and D.Harter, ‘Single-Mode Amplifiers and Compressors based on Multi-ModeOptical Fibers’, U.S. Pat. No. 5,818,630) (incorporated by referenceherein) enable the amplification of optical signals to levels whereapplications such as micro-machining and laser marking become possible.See A. Galvanauskas et al., U.S. patent application Ser. No. 09/317,221(incorporated by reference herein). Since laser marking andmicro-machining are dependent on the supply of high peak power pulses,it is advantageous to use such fiber amplifiers for the amplification ofnanosecond regime (ns) pulses, as supplied, for example, by micro-chiplasers or general Q-switched sources. In conjunction with large-corefiber amplifiers, these ns pulse sources can be amplified to pulseenergies in the hundreds of microjoules (μJ). Such fiber amplifiersystems can operate as direct replacements for Nd-based solid statelasers in any micro-machining or marking application.

The use of diffraction limited fiber multi-mode amplifiers allows asignificant improvement in the power density to be delivered to a targetcompared to non-diffraction-limited multi-mode amplifiers as describedfor example in B. Desthieux, Appl. Phys. Lett., vol. 63, No. 5, pp.586-588 (1993)). Note that in this early work the use of electricallydriven semiconductor lasers for the generation of short optical seedpulses to high power amplifier chains, albeit with a final multi-modepower amplifier, was described.

The use of electrically driven semiconductor lasers for amplifier chainswas later reiterated by Grubb et al., U.S. Pat. No. 6,151,338. A varietyof complex fiber amplifier implementations for use in micro-machiningapplications again reiterating the use of electrically drivensemiconductor seed lasers was recently also suggested in U.S. Pat. No.6,433,306 to Grubb et al. In U.S. Pat. No. 5,892,615 to Grubb, the useof bent single-mode amplifier fibers has been suggested; the difficultyin making such single-mode amplifiers has also been a limitation in suchhigh power fiber amplifier systems.

Advanced micro-machining or micro-structuring applications are enabledby amplification of ultrafast optical pulses in fiber media. Ultrafastoptical pulses are generally characterized by a pulse width of less than50 ps; conveniently, chirped pulse amplification is implemented toenable the amplification of such pulses to the μJ-mJ energy range.Generally, chirped pulse amplification systems use a nearbandwidth-limited seed pulse source, which is temporally stretched (i.e.chirped) in a pulse stretcher before amplification in a power amplifier.After amplification, the pulses are recompressed to approximately thebandwidth limit using a pulse compressor. In Ser. No. 10/606,829(incorporated by reference herein) the present inventors disclosed howto make such systems reliable and compact by utilizing nonlinear fibergrating stretchers with single pass large-core polarization maintainingamplifiers. In this invention, we disclose how to utilize telecommcomponents similar to those in manufacturing an ultrafast source forthis laser as is disclosed in IMRA Ref No. IM-108 (incorporated byreference herein). In Ser. No. 10/608,233 (incorporated by referenceherein) we disclose how to down count the rep-rate utilizing lithiumniobate modulators which are telecomm components. In this invention wewill disclose practical problems and solutions utilizing AOM for thesedown counters. AOM for fiber amplifier systems were first disclosed inU.S. Pat. No. 5,400,350 (incorporated by reference herein). In thisinvention we disclose how to solve or utilize the problem of spatialdispersion caused by the AOM.

Recently, modular, widely tunable fiber chirped pulse amplificationsystems were described by M. E. Fermann et al., ‘Modular, High EnergyWidely Tunable Ultrafast Fiber Source’, U.S. patent application Ser. No.09/576,772, (incorporated by reference herein) which increase thespectral width of ultrashort pulses in amplifiers by parabolic pulseamplification. These pulses can then be recompressed to pulse widthsshorter than the input pulses. In Ser. No. 10/437,057 (incorporated byreference herein) use of self-phase modulation in positive dispersionamplifiers with sources that have variable rep rates was described. Herewe disclose how to utilize such systems in industrial FCPA systems. Thisleads to a careful match of the spectral components of the amplifiersand the oscillators as well as careful control to maintain output pulseproperties. More recently parabolic amplification has been applied toFCPA systems in 2003/0156605.

Stabilization of gain against environmental variation is a key task foran industrial laser application. Most rare earth-doped fiber has anarrow absorption band where the wall-plug efficiency is high. InEr-doped fiber, usually a wavelength-locked 980 nm pump diode is used inorder to avoid variation of the emission spectrum of the diode overtemperature. (See, U.S. Pat. Nos. 5,563,732 and 6,335,944 B1, bothincorporated by reference herein). The wavelength locking can beprovided by partial feedback with a fiber Bragg grating. The temperaturesensitivity of a silica based fiber Bragg grating is typically in therange of 10 pm/C, whereas a GaAs-based quantum well chip has sensitivityin the range of 0.3-0.5 nm/C. On the other hand, this concept isavailable if the absorption spectrum of the gain fiber is not widelyvaried over the required temperature range. Also this concept isappropriate for Er-doped fiber where the absorption spectrum isrelatively stable over the industrial temperature range of 10-40 C. Thepump wavelength and gain over temperature are passively set at apredetermined value.

These methods, however, have been only demonstrated for a pump diodewith a single mode fiber coupler with the fiber Bragg grating written ina single mode fiber. Thus far, no embodiment of these methods has beendescribed for high power diode bundles with a multimode fiber coupler.Difficulties lie in coupling cavities in stable coherence collapseregime with a multimode fiber pigtail and homogeneous Bragg gratingfabrication in multimode fiber.

When effective wavelength locking is not available, an active regulationof the pump diode current or temperature is an alternative, bymonitoring the pump wavelength. However, this solution faces a limit ifthe temperature-dependent variation of the gain fiber absorption islarge, so that wavelength stabilization for the pump chip alone does notguarantee the required stable operation of the laser. An exactmonitoring of the absorption spectrum of the gain fiber over temperatureis required.

Thus the conventional concept described above is not applicable forstabilization of the gain fiber for stable output over a widetemperature range where: 1) wavelength locking using a device, e.g., afiber Bragg grating, with a low temperature sensitivity in comparisonwith the pump device is not available, 2) the absorption spectrum of again medium has a large variation over temperature, so that a passivelyset wavelength does not provide the required stability of gain, 3)temperature sensing of the gain fiber is necessary for accurate gainstabilization.

The prior art device for extracting one or more pulses from a series ofpulses produced by a high repetition-rate laser is an acousto-opticdeflector. A system that utilizes such a pulse selector formicromachining is given in U.S. Pat. No. 6,340,806. This means workswell with lasers that have very limited spectral bandwidths; typicallylasers with pulse duration in excess of about a nanosecond. However, thewide spectral bandwidth of the output of an ultrashort-pulse laserresults in the spatial separation of the various wavelength componentsof such a pulse when subjected to the dispersive Bragg grating of anacousto-optic deflector. This is a well known effect, and cavity dumpingpulses from ultrafast oscillators or from regenerative amplifiers asshown in (T. B. Norris “Femtosecond Pulse Amplification at 250 kHz witha Ti:sapphire Regenerative Amplifier and Application to ContinuumGeneration: Opt. Lett. 17, pp. 1009-1011 (1992)) are performed by doublepassing the AOM with a curved mirror that brings all the components backto the same point. Thus the dispersion becomes corrected. This meanscould be used here, however the system would not be compact. The instantinvention compensates for the dispersion, with the additional advantageof incorporating means to compress the chirped pulses to shorterduration.

SUMMARY OF THE INVENTION

The following topics are covered in this application.

-   -   1) Functional segmentation of opto-mechanical components into        modular devices to produce manufacturable industrial laser        systems with Telcordia-grade quality and reliability.    -   2) Polarization fidelity within and between modules    -   3) Provision for tap units for test, monitoring or feedback    -   4) Spectral matching of oscillator to amplifier    -   5) Selection of the length of an amplifier to cut ASE at the        lasing wavelength    -   6) Active stabilization of the optical performance of gain fiber        in a laser or amplifier. The stabilization is realized by        actively adjusting the pump source wavelength by changing the        source temperature in order to match pump wavelength with the        absorption spectrum of the gain medium. The temperature        dependent spectrum in the gain fiber is cloned in the same type        of fiber, and thus used as a monitor. Accurate control of the        gain performance over a wide range of operating temperatures is        possible implementing this method.    -   7) Extraction of one or more chirped pulses from a series of        such pulses using an acousto-optic deflector, and compensation        for detrimental effects on the spatial characteristics of the        extracted chirped pulse, caused by dispersion in that deflector.

The invention thus relates to the technologies necessary to overcome theabove problems and limitations of the prior art, to build a fiber-basedchirped pulse amplification laser system suitable for industrialapplications, in a modular and compact laser design with all modulesreplaceable. The modules are designed and manufactured to telecomstandards and quality.

Environmentally stable laser design is crucial for industrialapplication. An industrial laser system can be, for example,characterized by an output power variation below 0.5 dB over anenvironmental temperature range from 0 to 50 degrees Celsius, and bycompliance with the vibration, thermal shock, high temperature storageand thermal cycling test criteria in Telcordia GR-468-CORE andGR-1221-CORE. This target can be achieved by functional segmentation ofthe components and packaging the modular device with Telcordia-qualifiedpackaging technology. Before the modules are assembled into a system,they are tested and assembled separately.

Included in the modules are tap units that allow taking out signalsalong the propagation path in an integrated design. This is necessaryfor the optimization of each module as it is assembled, and important inthe spectral matching along the chain of modules.

Polarization units are provided to prevent the buildup of side-pulsesfrom orthogonal polarization light.

The acousto-optical down counter module can be designed to operate as abandwidth filter. For further modulation of the signal an additionalpulse extractor can be included near the end of the output. This unitsuffers from dispersion due to the large bandwidth of the pulse. Thecompressor can be used to correct for this dispersion as disclosedhereafter.

The invention also relates to a means to extract one or more chirpedpulses from a series of such pulses using an acousto-optic deflector,and to compensate for the detrimental effects on the spatialcharacteristics of the extracted chirped pulse caused by dispersion inthat deflector. An important aspect of this system is to manage thespectrum of the pulse in the system while maintaining the ability tocorrect for dispersion and compress the pulse back to the femtosecondregime. Two principal embodiments will be described. The first is thecase where the spectral content of the seed pulse is small. In this casea nonlinear amplifier may be employed for the generation of additionalspectrum while spectral filtering is employed to obtain a compressiblepulse. The second case is where the spectrum from the source is largerthan necessary. Nonlinear affects can be limited in the amplifier chainin this case, while spectral filtering is again employed to obtain acompressible pulse. An additional attribute that is necessary for manyapplications is the reduction of the ASE at the output. Specificamplifier designs are used to cut the ASE at the output wavelength. Thecompressor can be used as an optical spectral filter to this end.

Once gain performance is attained, a method for active stabilization ofthe optical performance of the gain fiber in a laser or amplifier isdisclosed to maintain this performance. The present invention stabilizesthe temperature dependent absorption of a gain fiber over a wideenvironmental temperature variation by an active feedback loop. A pieceof fiber, optically identical with the gain fiber itself, is used as aspectral filter for monitoring the emission spectrum of the pump diode.The absorption spectrum of the filter fiber follows that of the gainfiber if both fibers are packaged so that the fibers are in proximity toeach other. The transmission of the pump light through the filter fiberclones exactly the absorption characteristics of the gain fiber at agiven package temperature. The temperature of the pump diode iscontrolled by a feedback loop such that the transmission through thefilter fiber is maintained at the minimum. Importantly, the filter fiberfunctions as an active temperature sensor of the gain fiber. Precisespectral control of the gain at any fiber or package temperature canthus be realized.

As mentioned above, an important field of use for this system is inmicromachining. An additional feature needed for this application fieldis the capability to start and stop the pulse stream while moving thetargeted material in place. One method to do this is to control the downcounter. However, this leads to problems with gain stabilization in theamplifier and excessive ASE on target. These problems have beenaddressed in A8699 “Method and Apparatus for Controlling and ProtectingPulsed High Power Fiber Amplifier Systems” (filed concurrently herewithand incorporated by reference herein). However, another means to stopthe pulse stream is to utilize an optical switch at the output.

The invention extracts one or more chirped pulses from a series of suchpulses using an acousto-optic deflector, and compensates for thedetrimental effects on the spatial characteristics of the extractedchirped pulse caused by dispersion in that deflector. The instantinvention has the additional advantage that the means to compensate fordispersion in the acousto-optic deflector can be used to compress theduration of the chirped pulse. This is accomplished by placing the AOMin proximity to a grating compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a FCPA with nonlinear amplifier and 8 modules:oscillator, FI/attenuator, nonlinear amplifier, stretcher, preamplifier,down-counter with isolator, power-amplifier and compressor.

FIG. 1A illustrates an exemplitive oscillator.

FIG. 1B shows an example of a co- and counter-pumped fiber amplifier,which may operate as a non-linear amplifier or power amplifier.

FIG. 1C shows an example of a side-pumped fiber amplifier, which mayoperate as a non-linear amplifier or a power amplifier.

FIG. 2 illustrates a package of a telecomm-type attenuator and tap-pointmodules.

FIG. 3 shows the optical layout for a down-counter module. This showsthe ability to add additional components in the collimated beams beforeand after the AOM.

FIG. 4 shows the temporal performance of the down counter byillustrating the ˜10 ns window where a single pulse is selected from thetrain of pulses from the femtosecond oscillator.

FIG. 5( a) shows the spectrum from the oscillator and after firstfilter, isolator and attenuator module as illustrated in the FCPA blockdiagram in FIG. 1.

FIG. 5( b) is a component illustration of the filter, isolator andattenuator module.

FIG. 6( a) shows the spectrum from the nonlinear amplifier as a functionof pump diode current and ASE spectral output at peak current.

FIG. 6( b) is a component illustration of the isolator—attenuator modulebetween nonlinear amplifier and stretcher.

FIG. 7 shows the spectrum of pulses with self-phase modulationpropagating in a positive dispersion fiber.

FIGS. 8A and 8B are temporal and spectral profiles of the pulse afterthe stretcher as illustrated in the FCPA block diagram in FIG. 1.

FIG. 9 shows the spectrum after the power amplifier as illustrated inthe FCPA block diagram in FIG. 1.

FIGS. 10A and 10B illustrate autocorrelations of output pulses in the 5ps range and the 50 ps range. The spectrum of the output is shown inFIG. 10C.

FIG. 11 illustrates the FCPA with 8 modules: oscillator, FI/attenuator,stretcher, pre-amplifier, down-counter, FI, power-amplifier andcompressor.

FIGS. 12A and 12B respectively illustrate the spectrum from theoscillator and the spectrum after the filter module stretcher asillustrated in the FCPA block diagram in FIG. 11.

FIGS. 13A and 13B respectively show the spectrum after the preamplifierand the spectrum after the power amplifier as illustrated in the FCPAblock diagram in FIG. 11.

FIGS. 14A and 14B respectively show the spectrum after the compressorand the autocorrelation of compressed pulses for the FCPA illustrated inthe block diagram in FIG. 11.

FIG. 14C illustrates an example of the compressor.

FIG. 15 illustrates a generic scheme for active adjustment of the pumpdiode chip temperature by monitoring the transmission of the pump lightthrough gain fiber. The monitor fiber senses the internal packagetemperature.

FIG. 16 illustrates a preferred embodiment for accurate following of thegain fiber temperature by integrating the sensor fiber in physicalcontact with the gain fiber.

FIG. 17 shows an acousto-optic deflector illustrating the dispersivecharacteristics of an induced Bragg grating.

FIG. 18 shows a rudimentary two-pass chirped-pulsedispersion-compensated acousto-optic switch using transmission gratings.

FIG. 19 illustrates a lens-enhanced a two-pass chirped-pulsedispersion-compensated acousto-optic switch using transmission gratings;and

FIG. 20 shows an example of a lens-enhanced four-pass chirped-pulsedispersion-compensated acousto-optic switch using a reflection grating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents a block diagram of a modular all fiber-based chirpedpulse amplification laser system, modules that are first assembledseparately, tested and then assembled into a system. Each module isdesigned to be easily replaceable. The block diagram shows eightmodules: Oscillator, FI/Attenuator, Nonlinear amplifier, Stretcher,Pre-amplifier, Isolator/Down-counter, Power-amplifier and Compressor.Most of the modules have fiber in/out leads and can be characterizedseparately. After qualifying the module specifications, they are splicedtogether with standard PM-PM splices.

The modules are made up of a series of optical components that arepackaged by Telcordia-qualified technology adapted from thetelecommunications industry. FIG. 2 illustrates two typical packages.The first is the attenuator module. Linear polarized light is input intothe module from Fiber #1. The half-wave plate can be rotated to rotatethe linear polarization state. A polarizer follows the wave plate. Therelative angle of the linear polarization state to the polarizer willdetermine the attenuation of the signal that is launched in the outputfiber #2. The attenuator module can also have another function. Therecan be some degradation of the polarization state by nonlinear processesin the fibers, the use of short non PM fiber lengths or a slightmismatch of the polarization axis in splicing two PM fibers. Pulsespropagating along the second polarization state create satellite pulsesthat are undesirable. This problem is discussed in more detail in Ser.No. 10/627,069 (incorporated by reference herein). The wave plate isrotated so the polarizer rejects the light that is propagating in theunwanted polarization axis of Fiber 1. For this purpose a second 1/2wave plate may be placed following the polarizer to assure that thecorrect polarization of fiber 2 is launched. In certain criticallocations, it may be desirable to use an attenuator module in place ofsplicing two PM fibers.

The second module is a pigtailed tap point. This module, as illustrated,has two roles. One is to isolate the input module from followingmodules. This is accomplished with the isolator. The second purpose isto give a tap point. The tap point allows a small of amount of thesignal to be picked off as a diagnostic. The pickoff plate typicallyreflects a fraction of light off its front surface and the back surfacecan be anti-reflection coated to reduce the reflection. Often the frontsurface is uncoated and Fresnel reflection from the glass pick-off plateis utilized. This is a critical element in obtaining optimum operationof this system. Once the system is spliced together it would not bepossible to obtain a signal from each module in order for optimizationduring manufacturing or during service of the laser.

The first module 101 may be an ultrafast oscillator as is described inany of Ser. No. 10/627,069 “Polarization Maintaining DispersionControlled Fiber Laser Source Of Ultrashort Pulses” (incorporated byreference herein), IMRA Reference No. IM-108 “High Power Short PulseFiber Laser” (Incorporated by reference herein), and (IMRA ReferenceNumber IM-106 “High power ultrafast fiber MOPA source and itsapplication in frequency metrology” (incorporated by reference herein).FIG. 1A, reproduced from IM-108, shows a preferred arrangement of thisoscillator. The oscillator is based on two sub-modules, the saturableabsorber module and a pump laser module, a gain fiber and a fibergrating output coupler. It is clear that this module can be preassembledand tested before integration. The spectral output is shown in FIG. 5(a). The oscillator produces a 12 mW output with 1 amp. pump diodecurrent (34° C.). The repetition rate is 48 MHz. The center wavelengthis 1048.8 nm, and the bandwidth is 8.6 nm.

The FI/attenuator module follows the oscillator. The particularcomponents are shown in FIG. 5( b). It is a combination of theattenuator and tap point packages shown in FIG. 2. Rotating the secondisolator 502 relative to the first isolator 501 attains the attenuation.The half-wave plate 504 rotates the polarization stage to match the axisof the PM fiber. The filter 503 has a spectral width of 10 nm and iscentered at 1040 nm. Since the spectrum of the oscillator is centered at1048.8 nm, the spectrum of the transmitted light is narrowed to about 2nm and the peak is shifted to about 1043 nm. It is very desirable tohave this filter between the oscillator and the first amplifier in orderto modify the spectrum of the oscillator. Other types of filters such asfiber bragg gratings can be utilized in this location as is pointed outin IM-108. This module is used to attenuate the output to 1.4 mw. InFIG. 5 (b) it can be seen that two isolators are used. This has beenfound necessary in order to give sufficient isolation of the oscillatorfrom the first amplifier. 50 db isolation is ideally desired,however >35 db is normally sufficient. This level of isolation requirestwo isolators at this time.

The next module is the nonlinear amplifier. It is nonlinear due to thefact the pulse is not temporally stretched so that the amplificationtakes place with high intensity and thus significant self-phasemodulation. Typical amplifier configurations are shown in FIG. 1B (aco-propagating and counter-propagating pumped arrangement) and FIG. 1C(a side-pumped arrangement), although the precise configuration can beselected from many known amplifier designs. The spectrum at the outputof this amplifier is shown in FIG. 6( a). For higher pump currents thespectral width is over 20 nm. Thus in this nonlinear amplifier thespectral width has been increased by self-phase modulation by more thana factor of 10, from about 2 nm over 10 times to greater than 20 nm. Theamplifier is a Yb-doped cladding pumped fiber that is 4 meters long. InFIG. 1C, It is side-pumped with counterpropagating pumping. Even at thelowest current the spectrum has been broadened by self-phase modulation.At the higher current levels, the spectrum is typical for self-phasemodulation propagating in a fiber with positive dispersion. Comparethese spectra to that shown in FIG. 7, reproduced from Govind P.Agrawal, Nonlinear Fiber Optics (Academic Press, Inc. New York, 1989).

A nonlinear Yb amplifier with positive dispersion, usable in thisinvention and which has been utilized for pulse amplification of asubstantially unchirped pulse that is significantly spectrally broadenedduring the amplification and which can be pulse compressed afteramplification is disclosed in Ser. No. 10/437,057 “Inexpensive variablerep-rate source for high-energy ultrafast laser” (incorporated byreference herein). In these systems, highest gain and efficiency are notthe predominant concern as in the case of other amplifiers. The gain ofabout 100× in this stage is rather low for a fiber amplifier. The focushere is to obtain the highest pulse energy in a pulse that can becompressed.

Another feature of this amplifier is that the ASE peak is separated fromthe signal peak. Normally an amplifier fiber is optimized for efficiencyby uniform pumping of the fiber length. In this case, the gain peak andASE peak overlap. To move the gain peak to longer wavelengths and theASE peak at the output to shorter wavelengths then nonuniform pumping isutilized. In erbium and ytterbium amplifiers there is more loss atshorter wavelengths from unexcited ions. There are more unexcited ionsin an amplifier if high concentrations of active ions are utilized.There are fewer unexcited ions when the excitation levels are higher.High excitation thus reduces the loss at shorter wavelengths. Thereforethe gain peak can be moved away from a longer laser wavelength bypumping an end of a fiber to high excitation levels. The highestconcentrations of active ions are utilized while maintaining highefficiency of the fiber. In cladding pumped amplifiers, the corediameter is made large and the cladding diameter is made as small aspossible while coupling the pump diode light into the fiber. This designalso reduces the energy density in the fiber due to the large corediameter and all of the gain can be concentrated at the output end ofthe fiber. This optimizes the pulse energy due to the reduction of thenonlinear effects in the fiber. However, if additional nonlinearity isdesired the core diameter can be reduced or the laser diode can bedetuned off the absorption peak of the amplifier and the gain is mademore uniform. The fiber length is optimized for output power at thelasing wavelength. A longer length can be used with a small loss inefficiency to further shift the peaks apart, particularly for backwardpropagating ASE. Other components after the amplifier can be used as afilter to separate the ASE from the signal. The final compressor can beused for this purpose.

The module between the nonlinear amplifier and the pulse stretcher isshown in FIG. 6B. It contains an isolator 601 and a polarizer 604 withhalf-wave plates 602, 606. With nonlinear optical processes,polarization of the pulse can be altered by nonlinear polarizationrotation. Since the fiber is birefringent, light propagating along theother polarization axis of the fiber travels at a different speed andcan interfere with the main pulse in subsequent stages. For good pulsequality it is thus necessary to maintain polarization fidelity withpolarization elements between the different stages. Thus, this modulemay be inserted between other modules in the system as necessary. Thismodule is a combination of the attenuator and tap point module as isshown in FIG. 2.

The stretcher is a fiber that is single mode, PM and 200M long. Itstretches the pulse by >200 times to 120 ps as is shown in FIG. 8A.Other temporal pulse stretchers can be utilized such as the nonlinearfiber grating that is disclosed in Ser. No. 10/606,829 and 10/608,233(incorporated by reference herein).

After the stretcher is the preamplifier. This amplifier, like thenonlinear amplifier, is Yb-doped cladding pump fiber that is 4 meterslong. It is side-pumped with counterpropagating pumping. Thus the designis the same as the nonlinear amplifier, but as the pulse is nowstretched, nonlinear affects are avoided.

The down-counter module follows the preamplifier. The optical layout ofthe down counter is shown in FIG. 3. The principal component of the downcounter is an AO modulator 350. Its function is illustrated in FIG. 4.The AO modulator has a window of about 10 ns. Its purpose is to selectone pulse out of the train of pulses from the mode-locked oscillatoroperating at 50 MHz. The down counter electronics determines how often apulse is selected and hence the repetition rate of this amplifier. Forexample if one out of every one hundred pulses is selected from the 50MHz oscillator, then the amplifier repetition rate is 500 KHz. Thedown-counter module can have other functions. The use of two lenses 360,370; 380, 390 before and after the AOM gives a section of collimatedlight. At these points other components can be placed such pick-offwindows 325, attenuators or filters 335. FIG. 8B shows the spectrumafter the down-counter module. It can be seen that the wings of thespectrum and the large modulations have been removed. This is desirablesince the wings cannot be compressed and the large modulations wouldcause self-phase modulation in the final amplifier, since self-phasemodulation is related to the derivative to the light intensity. In thiscase an optical filter was not used. In this case the preamplifier wasused as a spectral filter. Its gain bandwidth is narrower than thespectrum from the nonlinear amplifier. Additional or alternativefiltering could be caused by the down counter and would be related tothe chromatic aberration from the lens system and the spectraldispersion in the AOM. In this case, the spectral throughput is adjustedby the alignment of the output fiber. The dispersion in the AOM will bedescribed in more detail later.

The next module is the power amplifier. Amplification is generallylinear here, since the pulse has been stretched before amplification,which lowers peak pulse energy. A single-pass, end-pumped amplifiermodule is typically used. The amplifier fiber is polarizationmaintaining with a 30-μm core diameter and 250-μn cladding diameter. Thelength is 3 m. The pump laser diode is operated at 14 W. It is typicallyend pumped in the counterpropagating direction. FIG. 1B showscounterpropagaing pumping with Pump II and copropagating pumping withPump I. The laser diode or an array of laser diodes is typically coupledinto a fiber bundle. It is found advantageous to use two fiber bundleswhere the connection or splice is on the outside of the laser enclosure.In this manner the fiber bundled diode can be replaced withoutcontaminants getting into the optics of the end-coupled unit. Fiberbundled ends are also not consistent in their output so replacing thebundle directly tends to misalign the end coupled unit. At this point,the amplifier generates 1.01 W output at 200 kHz repetition rate. Thespectral profile is shown in FIG. 9. Again the spectrum is narrowed fromthe amplifier. The amplifier also operates as a spectral filter. Thewings of the self-modulated pulse are not compressible so the filteringhelps the fidelity of the pulse.

The last module is the bulk-grating compressor. The design as shown inFIG. 14C is a folded Martinez-type configuration. Typically we use aconventional design compressor as is shown in U.S. Pat. No. 5,847,863.However by utilizing the retroreflecting mirror M4, a single grating canbe used. The grating is 12001/mm blazed at 1 micrometer and used at thelittrow angle. The output is 405 mW at a 200 kHz repetition rate. Thus a2 μJ pulse is generated. The duration measured by the autocorrelator is488 fs. This gives a 345 fs pulse width assuming a Gaussian pulse shapeas is shown in FIGS. 10A and 10B. The spectrum is shown in FIG. 10C.Again the spectrum is reduced where the grating compressor is operatingas a filter that filters out the wings that are not compressible. Itshould be noted that the spectrum has high frequency modulation due topolarization interference. This shows the importance of maintaining thepolarization fidelity in this amplifier. This leads to a longer pulsebackground as is shown in FIG. 10B.

A block diagram of a second embodiment is shown in FIG. 11. The majordifference in this embodiment is that the pulses are stretched beforethe first amplifier. Thus, in this design, nonlinear optical processesare minimized in the amplifiers. The advantage of this system is thatreproducibility from system to system is easier to attain. However, theseed bandwidth from the oscillator has to be broader after the filterand before the stretcher (12-14 nm compared to 3 nm). The spectrum ofthe oscillator and after the first filter module is shown in FIGS. 12Aand 12B. It is critical to have a filter between the oscillator and therest of the system. At this point the spectral characteristics of eachamplifier are not sufficiently the same to operate a system without afilter. The filter can be placed before the stretcher or after. If it isplaced before the stretcher, then an attenuator module is used with thismodule. If the spectrum is filtered before the stretcher it is necessaryto keep the amplitude of the input pulse sufficiently low to preventnonlinear effects in the stretcher from changing the spectrum. Theattenuator is used for this purpose. It should be noted that the highfrequency noise on the spectrum in FIGS. 12 a and 12 b is caused bypolarization imperfections. This illustrates the need for polarizationmodules between fiber sections. To fully prevent any high frequencynoise a polarization module could be placed at each PM to PM splice.However, this is an over-engineered design for this system. Since thelight at the wrong polarization accumulates in the system, polarizationmodules are placed primarily where it is convenient, such as part ofanother module.

The spectrum from the preamplifier and the power amplifier are shown inFIGS. 13A and 13B. It will be noted that no noticeable nonlinear affectsare taking place in the amplifiers. There is also some narrowing of thespectrum from gain narrowing. The spectrum of the output from thecompressor is shown in FIG. 14A with the autocorrelation shown in FIG.14B. As with the first embodiment, the pulse duration measured by theautocorrelator is 488 fs. This again gives a 345 fs pulse width assuminga Gaussian pulse shape. The output is 440 mW at a 200 kHz repetitionrate. Thus a 2.2-μJ pulse is generated.

In order to be able to recompress the pulses, the spectrum of the outputmust be limited to 7-9 nm when a fiber stretcher and a bulk-gratingcompressor are used together. In order to get this spectral width theinitial width needs to be broader since the amplifiers have some gainnarrowing. In the case of the nonlinear amplifier, additional narrowingand filtering is necessary to get rid of the spectral wings that are notcompressible. In the case of the nonlinear amplifier, the spectral widthwas near 20 nm after the amplifier while for the linear amplifier thespectral width after the preamplifier was about 9 nm.

Therefore, the nonlinear amplifier has an additional sensitivity on thefluctuation of output power. With additional output, there will beadditional self-phase modulation and the spectral width will increase.Thus, the gain in the nonlinear amplifier needs careful control sincethe spectral characteristic will change with gain. In the firstembodiment using the nonlinear amplifier, such careful control is neededhowever, control is also desired in the second embodiment. In this casethe amplifier uses v-groove pumping (as disclosed in U.S. Pat. No.5,854,865, incorporated by reference herein) in combination with adouble-clad fiber for coupling high intensity pump light into the fiberusing a high power broad strip diode chip. However, a drawback of thisscheme is the lack of accurate locking of the pump wavelength. Due tothe v-groove pumping directly from the laser diode, incorporation of afiber Bragg grating is not feasible. A narrow band dielectric coating onthe chip facet will not solve this problem. Further, even though thechip is kept at a predetermined set temperature, the actual chiptemperature varies slightly due to bias current and the heat sinktemperature.

Furthermore, in a system comprised of rare earth-doped fiber, such asYb, with a high temperature sensitivity, the mismatch of the absorptionspectrum with the pump spectrum over the environmental temperature rangeis a significant issue in stabilizing the laser or amplifier output. Thetemperature dependent gain of rare earth doped fiber results both fromspectral variation of the absorption cross section and from theradiative recombination rate over temperature. In regard to the spectralabsorption cross section, a spectral shift and broadening/narrowingoccur due to thermally populated optical transition states upon fibertemperature change. In an Er-Yb system an absorption change up to −2 dBhas been observed over 25 to 60° C. fiber temperatures by the shift andbroadening/narrowing effect of the absorption spectrum. (See, K. W. Quoiet al., Journal of Lightwave Technol. Vol. 10, 1992, p 847) In fact suchtemperature dependent absorption in a rare earth-doped fiber can beexploited for temperature sensing. (See, K. W. Quoi et al., Journal ofLightwave Technol. Vol. 10, 1992, p 847)

If a Yb-fiber is combined with an unlocked pump wavelength, which ismostly the case for v-groove (side) pumping or multimode diode pumping,it is difficult to build a reliable laser or amplifier with a highoutput stability over a wide range of operating temperature. Anabsorption change of 10% over 10-50° C. of fiber temperature can beeasily observed in a double clad single-mode Yb fiber with a >−10 dBabsorption length at 976 nm. This temperature sensitivity is especiallycritical for an oscillator where the gain fiber is not in a saturationregime. A precise match of the pump spectrum with the absorptionspectrum over all operating conditions are necessary. The narrowabsorption peak at 976 nm, which has the highest wall plug efficiencyfor Yb pumping, is sensitive to the pump wavelength distribution. Aslight variation of the absorption spectrum, especially a spectral shiftover temperature, will easily result in an unacceptable variation of theoutput. The full-wave half-maximum of the 976 nm absorption band isabout 7 nm, thus a control of the pump wavelength distribution within1-2 nm is necessary in order to achieve system stability within +/−2%.

The need for control is not limited only to the spectral distribution,but also temperature and diode current. One way of implementing suchcontrols is to include a feedback loop as disclosed below. In this way,broadening or narrowing of the absorption efficiency is reproduced inthe monitor fiber as the fiber temperature changes. Compensation for theefficiency change can be implemented by adjustment of the pump diodecurrent.

This method of controlling pump dynamics by monitoring the output powerof an amplifier with a low repetition rate (for example, <200 kHz) isanot a proper solution if pulse-to-pulse stability is important. Such afeedback loop requires at least two sequential pulses detected beforeadjusting the pump. The invention, for monitoring the pump spectrum,disclosed herein is based on continuous wave mode. Depending on thespeed of the feedback loop electronics, pulse-to-pulse fluctuation canbe eliminated completely.

The absolute temperatures of the cores as between the gain and monitorfibers can deviate from each other due to the fact that the gain fiberis subject to high optical pumping. However, the method disclosed hereinis based on the relative change of the fiber optics characteristics.Thus an absolute temperature offset does not detract from the concept ofthe invention.

FIG. 15 represents this further embodiment of the invention. The pumplight from diode source 101 is tapped at 102 for a feedback loop 105.Typically, the tap ratio needs to be −20 dB. The monitor port is splicedwith a piece of Yb fiber that is optically identical to the gain fiber.The length of the monitor fiber 103 can be conveniently adjusteddepending on the electronics, including the photo diode 104. Theelectronics regulates the diode temperature so that the transmissionthrough the monitor fiber is at a minimum, corresponding to the optimalmatching of the pump wavelength distribution within the absorption bandof the gain fiber 106. The monitor fiber can be packaged inside thesystem so that the temperature of the monitor fiber is in closeproximity to that of the gain fiber.

A tap between the laser diode and the amplifier is not always possible.This is particularly true with the V-groove side-coupling module. Thereare alternative methods to obtain diode pump light for the monitor fibersuch as to pick off scattering or spurious reflections in theside-coupled unit. Bend losses from the amplifier fiber can be used onthe initial section of the amplifier. Another alternative is to use thegain fiber itself as the monitor fiber. The tap is then the light thathas passed through the amplifier. For controlling the nonlinearamplifier in embodiment one, the tap point in the filter, isolator andattenuator module shown in FIG. 5B can be used. However, the light willbe reflected off in the opposite direction from the reflection opticsand an additional window is necessary. It should be mentioned again thatthe nonlinear amplifier is pumped in a counter-propagating direction sothis pump light has traveled through the nonlinear amplifier fiber.

FIG. 16 shows an exemplary embodiment of the invention for more precisesensing of the gain fiber temperature. The monitor fiber is brought intothermal contact with the gain fiber. The proper length of the monitorfiber can be adjusted by fusion-splicing it with an undoped transmissionfiber.

In a further embodiment, an AOM is added near the end of the system forselecting pulses for applications such as micromachining. Althoughcontinuous pulse output is sometimes acceptable, the user often requiresthat one or more pulses be selected and delivered to the applicationupon demand. As aforementioned, for nanosecond or longer pulse lasers,an acousto-optic deflector or modulator (AOM) can usually be used toselect a pulse by deflecting it to the required application. However,the dispersion of an AO Bragg grating, along with the high spectralbandwidth of a chirped pulse, prohibits a simple solution for ultrashortpulse lasers.

As shown in FIG. 17, the constituent wavelength elements of a chirpedpulse deflected by a Bragg-grating are disperse spatially, therebyresulting in a so-called spatially chirped pulse, which is detrimentalfor most applications. Also, it is generally required that there exist ahigh contrast between the selected and delivered pulses and thealternate state. Configurations, where the selected pulse is undeflectedand the deflected pulses are discarded, is generally unsuitable for mostapplications as it has unacceptably low pulse contrast ratio.

In the principal (first) embodiment of the invention, utilization of thedispersive affect of an AOM Bragg grating on a chirped pulse operates asan improvement, rather than a limitation, to the design of an ultrashortpulse selector, with the advantage that this design lends itself tosubsequent compression. The compression of a chirped pulse relies ondispersion of its constituent wavelengths to affect a differential pathlength delay between its leading and trailing components (see FIG. 18).The dispersive elements in a traditional pulse compressor areconventional surface-ruled diffraction gratings, although transmissiongratings are also used. A novel aspect of this invention is tosubstitute one or more, but typically just the initial, diffractiongrating of the pulse compressor with an AOM. Choosing to replace thefirst grating with the AOM, rather than any other, is driven by twoconsiderations. One, mitigation of optical damage in the acousto-opticmedium, as the 1st element of the compressor is subjected to thestretched pulse, which has lower peak power as compared to that at the4^(th) element. Two, minimization of the physical size of the AO medium,as the aperture of the 2^(nd) and 3^(rd) dispersive elements of thecompressor must be larger than the 1^(st) or 4^(th) so as to accommodatethe spatially chirped pulse. Generally, the compressor is designed sothat the optical path passes multiple times through each component tominimize cost and provide a compact configuration.

Although the feature of compression in this invention is usually of highvalue, if uncompressed pulses are required, the subsequent gratings maybe “misaligned” to simply spatially reconstruct the pulse withoutcompression.

The invention applies to compensation of dispersion of chirped pulsesfor any acousto-optic device. However, certain practical designconsiderations are important to the effective operation of the AOM asused herein. In order to attain high dispersion to match typicalgratings, a relatively slow acoustic wave and high frequency driver isrequired. The optimum material for use in the visible and near infraredis tellurium dioxide, as it supports a slow shear wave that generates agrating applicable to this invention. Although this shear wave has avelocity appropriate to this application, it suffers from highattenuation that can limit the useful aperture of devices that use it.For devices that require an aperture of three or more millimeters, theAO crystal design may utilize an off-axis shear wave configuration. Thisdesign produces nearly the same low velocity wave, but withsubstantially lower attenuation.

A specific design can be calculated by starting from the BraggDiffraction equation for the AOM.

λ_(p)=(2d/n)sin θ

where

-   -   λ=photon wavelength    -   d=grating period    -   n=diffraction order, and    -   θ=diffraction half-angle.

Since the diffraction angle will be small (<<10°), use the paraxialapproximation, i.e. sin θ≈θ. Thus,

λ_(p)≈(2d/n)θ.

Now differentiating the above

Δλ_(p)≈(2d/n)Δθ

The Bragg grating generated in an AO deflector or modulator is

d=λ _(S) =c _(S) /f _(R)

where

-   -   λ_(S)=sonic wavelength    -   c_(S)=sonic velocity    -   f_(R)=radio frequency

Combining the above two equations, and solving for Δθ yields

Δθ=nΔλ _(p) f _(R)/2c _(S)

For a typical four-pass holographic transmission-grating compressor,which uses an 800 groove/mm, grating, and other typical parameters foran AOM, assume the following parameters:

n=1Δλ_(p)=0.02 μmf_(R)=350 MHzc_(S)=617 m/s (TeO₂ shear wave)Δθ≈0.0056 radians≈0.33°

The dispersion from the 800 groove/mm grating corresponding to 0.02 μmis about 1°, thus we include a simple cylindrical lens 191 with angularmagnification of 3 in the switch design. This design is shown in FIG.19. Still alternately a telescope may be used. Alternately, reflectiongratings (with concomitant lower diffraction efficiency, and cost) canbe used.

However, even using the off-axis shear wave, attaining the requiredangular dispersion is difficult at a sufficiently low crystal drivingfrequency where attenuation of the ultrasonic wave is not excessive. Inorder to practically produce sufficient angular dispersion to matchgratings in a typical compressor, the use of additional optical elementsis advantageous. This can be accomplished with a negative (concave)cylindrical lens (see FIG. 19), a telescope (not shown), or otheroptical means, such as, but not limited to, a reflective element, of theappropriate power. The use of these ancillary optical elements with theAOM is an additional embodiment of this invention. The selectioncriteria to optimize the performance of this pulse extractor-compressormust consider driving frequency (wave attenuation), shear wave angle(attenuation and dispersion), and dispersion matching optics (physicalsize and complexity). Other configurations of switch and compressor areuseful and advantageous, particularly where a higher compression factoris desired, such as, but not limited to a four-pass reflection-gratingconfiguration as shown in FIG. 20. Double passing the AOM such as usedin a regenerative amplifier is also suitable.

These embodiments implement a principal object of the present inventionto extract and redirect a pulse or group of pulses from an essentiallycontinuous series of pulses. The AOM deflects the extracted pulses togratings or other dispersive means, which complete the spatialreconstruction of the pulse, with the option for compression, anddirects the pulse or pulses to the user application. Unextracted pulsesare not be delivered to the user application, but may be directed toother diagnostic means so as to establish the operating condition of thelaser system prior to extraction of pulses for the application. Thisscheme is of value to measure performance of the laser output prior todelivery of the pulses to a critical application.

1. A pulsed laser system, comprising: a signal source; a stretchertemporally stretching an output of said source; a fiber amplifier; acompressor recompressing an output of said amplifier; an AOM selectingoutput pulses from said amplifier; wherein each of said signal source,said stretcher, said fiber amplifier, said AOM and said compressorcomprise a pre-tested module, and said system is configured with atleast said signal source, said stretcher, said fiber amplifier and saidAOM optically connected via fiber splices; at least one tap unit withinor between ones of said modules, including means for picking off aportion of said signal for measurement of an optical pulsecharacteristic within or between said modules, wherein said at least onetap unit provides a small signal output to a diagnostic means, saidsmall signal output being provided during manufacturing or service ofsaid pulsed laser system; and an attenuator module disposed between saidsignal source and said stretcher, said attenuator controlling anamplitude of a pulse emitted from said source so as to preventnon-linear effects within said stretcher from altering a pulse spectrum.2. The pulsed laser system as claimed in claim 1, wherein said filter ispackaged within a filter-attenuator (F1/Attn) module.
 3. The pulsedlaser system as claimed in claim 1, wherein said signal source comprisesa fiber oscillator, and said at least one tap unit provides a smallsignal output for measurement of said oscillator spectrum.
 4. The pulsedlaser system as claimed in claim 1, further comprising: at least oneisolator module between said signal source and said fiber amplifier forproviding at least 35 dB isolation therebetween.
 5. The pulsed lasersystem as claimed in claim 1, wherein said at least one tap unitprovides a small signal output representative of pump power from a pumpdiode source, said small signal output providing an input for a feedbackloop to regulate the pump diode source.
 6. The pulsed laser system asclaimed in claim 1, said system comprising at least one spectral filter,said at least one spectral filter being disposed externally to saidsignal source and said fiber amplifier, and either before or after saidstretcher, said filter having a wavelength sensitive characteristic thatmodifies a spectral shape of an output from said signal source andnarrows the spectrum of the signal source output, wherein at least oneof said tap units is located downstream of said filter and configured toenable measurement of said narrowed spectrum, and wherein said filter isdisposed before said stretcher.
 7. A pulsed laser system, comprising: asignal source; a stretcher temporally stretching an output of saidsource; a fiber amplifier; a compressor recompressing an output of saidamplifier; an AOM selecting output pulses from said amplifier; whereineach of said signal source, said stretcher, said fiber amplifier, saidAOM and said compressor comprise a pre-tested module, and said system isconfigured with at least said signal source, said stretcher, said fiberamplifier and said AOM optically connected via fiber splices; at leastone tap unit within or between ones of said modules, said tap unitconfigured to produce a relatively small optical signal to be picked offas a diagnostic during manufacturing or service of said pulsed lasersystem; and an attenuator module disposed between said signal source andsaid stretcher, said attenuator controlling an amplitude of a pulseemitted from said source so as to prevent non-linear effects within saidstretcher from altering the pulse spectrum.
 8. The pulsed laser systemas claimed in claim 7, wherein said small signal output isrepresentative of a signal spectrum within or between said modules. 9.The pulsed laser system as claimed in claim 7, wherein said at least onetap unit provides a small signal output representative of pump powerfrom a pump diode source, said small signal output providing an inputfor a feedback loop to regulate the pump diode source.
 10. The pulsedlaser system as claimed in claim 7, said system comprising at least onespectral filter, said at least one spectral filter being disposedexternal to said signal source and said fiber amplifier, and eitherbefore or after said stretcher, said filter having a wavelengthsensitive characteristic that modifies a spectral shape of an outputfrom said signal source and narrows the spectrum of the signal sourceoutput, wherein at least one of said tap units is located downstream ofsaid filter and configured to enable measurement of said narrowedspectrum, and wherein said filter is disposed before said stretcher. 11.A pulse laser system composed of a plurality of replaceable modules,comprising; an ultrafast oscillator module; a nonlinear amplifier modulereceiving the oscillator output and amplifying the signal whilebroadening its spectral width; an isolator module for eliminatingfeedback into the oscillator; a stretcher module for temporallystretching the output of said isolator/polarizer oscillator; a linearpre-amplifier module for amplifying the stretched output; a down-countermodule for controlling the repetition rate of the laser system; a poweramplifier module for amplifying the output of the down-counter module;and a compressor for temporally shortening the output of said poweramplifier module.
 12. A pulse laser system composed of a plurality ofreplaceable modules, comprising; an ultrafast oscillator module; astretcher module for temporally stretching the output of said oscillatormodule; a spectral filter module placed before or after said stretchermodule and external to said oscillator module, said spectral filtermodule optically connected to said ultrafast oscillator module andcomprising at least one spectral filter having a wavelength sensitivecharacteristic that modifies a spectral shape of an output from saidultrafast oscillator, including a spectral width of said output; atleast one pre-amplifier module downstream of said spectral filtermodule; a down-counter module for reducing the repetition rate of thelaser system; a power amplifier module for amplifying the output of thedown-counter module; and a compressor for temporally shortening theoutput of said power amplifier module.
 13. A pulse laser system,comprising the following components: a signal source; a stretchertemporally stretching an output of said source; a fiber amplifier; and acompressor recompressing an output of said amplifier; and an AOMselecting output pulses from said amplifier; wherein each of saidcomponents comprise a pre-tested module, and said system is configuredwith at least said signal source, said stretcher, said fiber amplifierand said AOM optically connected via fiber splices; at least one tapunit within or between ones of said modules, including means for pickingoff a portion of said signal for enabling measurement of a spectrumwithin or between said modules, and at least one spectral filter, saidat least one spectral filter being disposed external to said signalsource and said fiber amplifier, and either before or after saidstretcher, said filter having a wavelength sensitive characteristic thatmodifies a spectral shape of an output from said signal source andnarrows the spectrum of the signal source output; wherein at least oneof said tap units is located downstream of said filter and configured toenable measurement of said narrowed spectrum.
 14. An all-fiber chirpedpulse amplifier system composed of a plurality of modular opticalsubassemblies, comprising; at least an oscillator module, a stretchermodule, and an amplifier module, each subject to separate assembly andtest, at least said oscillator module, said stretcher module and saidamplifier module being coupled into the system by a fiber splice; meanslocated between respective ones of said modules for connecting testequipment for monitoring at least one of spectrum and pulse quality ateach location, at least to identify uncompressible pulse components, thenumber of said means for connecting test equipment is at least one, butless than the number of said modular subassemblies and means locatedbetween respective ones of said modules for improving fidelity of thepolarization state, where the number of said means for improvingfidelity of the polarization state is at least one, but less than thenumber of said modular subassemblies.
 15. A chirped-pulse amplificationsystem, comprising; an ultrafast oscillator module; a spectral filternarrowing a spectrum of a signal output from said oscillator; at leastone amplifier module; a down-counter module; a compressor module; andmeans between ones of said modules for improving fidelity of thepolarization state